This invention pertains to improving the performance of double-resonance circular-polarization (CP) homogeneous rf resonators for magnetic resonance imaging (MRI), for use especially where the frequency-diameter (fd) product is greater than 10 MHz-m and at least up to 60 MHz-m, possibly higher. It is related to hybrid birdcage (BC) coils, as disclosed by Edelstein et al in U.S. Pat. No. 4,680,548, and Crozier's 8-section variant, U.S. Pat. No. 5,642,048.
As is well known, the simple lumped-element models of the birdcage as disclosed initially by Edelstein and Hayes are of limited value in predicting tuning value components and mode frequencies except at rather low fd products—generally below about 15 MHz-m. The circuit model shown in FIG. 1 for the 8-section hybrid case, is perhaps the best simple model for use in commonly available linear circuit simulation software for the hybrid birdcage, also known as the band-pass birdcage. There, circuit nodes are identified as needed to define the model, and each axial inductive element is represented by the series combination of a mutual inductance LC to each adjacent rung with a transmission line (TRL) at each end. The initial model parameters of the TRLs (length, impedance, propagation velocity, and attenuation coefficient) are estimated by standard approximations from the physical dimensions of the coil. Then, mutual inductive couplings to adjacent rungs are estimated for the lumped couplings, and the lengths of the TRLs in the model are shortened appropriately. The characteristic impedance and propagation velocity are reduced somewhat, and the attenuation coefficient is increased, as discussed by Doty et al, JMR, 138:144-154, 1999. The 8-section model shown is easily extended to 12, 16 or higher numbers of sections. Note that the right-most ring inductive elements connect back to nodes 11 and 17 respectively.
For CE substantially larger than C1, the hybrid coil is essentially the balanced low pass (LP) birdcage, and the lowest mode frequency, m1, supports homogenous circular polarization. The LP BC is seldom selected for fd above 15 MHz-m, as the tuning becomes strongly sample dependent because of its relatively high conservative electric fields.
For C1 well over an order of magnitude larger than CE, the hybrid coil approximates the balanced high pass (HP) birdcage, and the highest (or possibly next to highest, if an end-ring mode is present) mode frequency, also called m1, supports homogenous circular polarization. Here, both the usable fd range and the homogeneity are improved by increasing the number of rung elements, and frequently 16 are used, though another option is to use two parallel bands per axial inductive element, as in the Crozier coil, for extended range and improved homogeneity for a given number of sections. In this case, 8 sections are generally sufficient for fd up to 50 MHz-m, especially if an insulated cross-over is added in each rung section, as disclosed by Doty in U.S. Pat. No. 6,060,882.
When CE is less than an order of magnitude larger than C1 but not much less than C1, the coil becomes the hybrid birdcage, with a more complex and closely spaced mode structure; and for that reason it has seldom been used, even though it can in principle extend the BC to somewhat higher fd products. However, for fd above 45 MHz-m, the circuit model is of limited accuracy (even with minor improvements, such as appropriate parasitic stray capacitances at the various nodes), and full-wave finite element software or trial and error is required to tune the coil. With some state-of-the-art full-wave software, the homogeneous and adjacent modes and field strengths may be predicted to within a few percent. This permits the high-pass-weighted hybrid BC (in which C1 is typically 5 to 30 times CE) to be a practical method of extending the range of the BC to higher fd products, as shown recently by Doty et al in a poster presentation, “An 11 cm, 500 MHz Hybrid Birdcage with Improved Tuning Range”, at the 13th ISMRM, 2005, in Miami. Some advantage may also sometimes be obtained if the LP capacitors (C1) are located at an intermediate position in the TRLs, according to the prior art, rather than at their ends.
Double-resonance techniques in MRI comprise but a small fraction of a percent of current MRI applications, primarily because MRI is usually interested in maximum resolution, which requires high signal to noise (S/N) and the S/N of protons usually exceeds that of the other available nuclides by more than an order of magnitude. The additional information available from spatially localized spectroscopy of nuclides other than 1H has driven most of the double-resonance MRI applications thus far, but these applications have been limited.
Recently, it has been shown that 13C may be hyperpolarized via novel dynamic nuclear polarization (DNP) methods which permit its S/N to be increased by up to four orders of magnitude (see, for example, Golman et al, PNAS, 2003). Moreover, this polarization may be transferred to other nuclides (including 1H) for dramatic increases in the S/N of other nuclides. Hence, it seems that DNP will stimulate strong growth in the applications of double-resonance MRI in large samples at high fields.
Most prior art double resonance MRI experiments for the fd range of 4-20 MHz-m have utilized orthogonal linear-polarization (LP) litz coils, as disclosed in U.S. Pat. No. 6,060,882. The lower end of the above fd range will continue to be best addressed by such coils, owing to their advantages in manufacturability, homogeneity, and S/N for this range, where sample losses are not strongly dominant. However, for fd above 20 MH-m for smaller coils, or above 10 MHz-m for larger coils, sample losses become strongly dominant relative to coil and capacitors losses, and rf eddy currents or dielectric resonance effects within the sample begin to seriously affect rf field homogeneity. Under these conditions, CP rf coils are preferable. Moreover, obtaining clean, homogeneous modes in double resonance MRI using orthogonal linear litz coils at fd above 15 MHz-m has been technically problematic.
Isaac et al (JMR, 89:41-50, 1990) presented a method of double-tuning the unbalanced LP BC by inserting an HF tank (trap) into each rung. This coil generates rather closely matched field profiles at the two frequencies, as it behaves essentially as an unbalanced LP BC at both the LF and the HF. However, it has excessive sample losses and poor S/N for HF fd greater than 10 MHz-m.
Fitzsimmons et al (MRM, 30:107-114, 1993) describe the use of an unbalanced LP BC for the LF inside a balanced HP BC for the HF. Here, the LF is still effectively limited to ˜10 MHz-m (for example, 31P up to 2 T for the human head), and the couplings between the coils make tuning extremely challenging. They partially address this latter issue by making the outer HP coil significantly longer and larger in diameter than the inner coil, which results in excessive HF ROI (region of interest) and sample heating. The field profiles of these separate coils are very poorly matched.
Vaughan et al (MRM, 32:206-218, 1994) have demonstrated the use of the TEM (transverse electro-magnetic) resonator for double resonance at HF fd up to 45 MHz-m. While this resonator is in principle capable of generating CP fields with matched field profiles, the technical challenges are daunting, arising from the nature of its closely spaced mode structure, which follows from the use of weakly coupled resonator elements. Consequently, the double-tuned TEM resonator has not been commercially successful.
Murphy-Boesch et al (JMR B 103:103-14, 1994) describe several 4-ring birdcages, including an LP-LP and an LP-HP, for the generation of CP fields at two separate frequencies. A similar approach was taken by Varian with their DT “millipede” coil, introduced commercially in 1999. The primary problem with 4-ring structures is that they generate strong, inhomogeneous rf fields within the sample outside the ROI—a manifestation of their absence of field profile matching. As a result, there is excessive sample heating and poor S/N. Another problem with 4-ring structures is that they have a large number of closely spaced parasitic inhomogeneous modes, which severely complicates tuning and renders simple circuit models essentially useless.
Shen et al (MRM, 41:268-275, 1999) improve upon Isaac's double-tuned (DT) LP BC and maintain rather well matched field profiles at the two frequencies (except near the traps). They show it can be used for fd up to 32 MHz-m and the tuning component values can be calculated quite accurately. Still, the topology is unbalanced LP at both frequencies and S/N is poor at the HF.
Matson et al (JMR, 139:81-89, 1999) describe a BC analogy to Vaughan's TEM coil, in which alternate rungs are tuned to two different frequencies in a balanced LP configuration. They achieve remarkable S/N at the LF in a 1.5 T head coil; but, as the topology is LP, the HF performance is poor. Also, since only 8 rungs are used for each frequency, B1 homogeneity suffers at both frequencies.
Fiat describes a DT coil tuning approach in U.S. Pat. No. 5,675,254 for linear coils that had been in wide-spread usage in commercial MRI and NMR coils since the mid-1980's. Shen suggests the use of multi-layer structures and electronic switches for multi-resonant NMR or MRI in U.S. Pat. No. 6,081,120. Hartman, in U.S. Pat. No. 6,366,093, discloses novel re-entrant cavities capable of generating linear rf magnetic fields of high homogeneity at one or two frequencies simultaneously with essentially equal field profiles for a narrow range of suitable conditions with respect to fd product and available space.
In summary, prior-art DT CP coil designs for large samples at high frequencies have generally exhibited some combination of (a) increased high-frequency (HF) sample losses, (b) degraded B1 homogeneity at one or both frequencies, (c) increased low-frequency (LF) coil losses, (d) widely differing rf magnetic field profiles at the two frequencies, or (e) severe tune-up and matching challenges. The novel coil in this invention addresses these issues and has improved homogeneity, sensitivity, channel isolation, symmetry, tunability, and manufacturability compared to the prior art.
The preferred, novel DT CP rf head coil disclosed herein behaves as a low pass (LP) birdcage at the LF and a high pass (HP) birdcage at the HF. It efficiently generates uniform circular polarization (CP) at both frequencies. It is advantageous for DT human head coils in which the HF is 1H from 1 T at least up to 4.7 T and probably higher. The improvements in this invention are achieved from the novel topology that, among other characteristics, results in two homogeneous modes, widely separated in frequency, with similar spatial profiles of the rf magnetic fields, both within the homogeneous region and beyond. At both the HF and the LF homogeneous modes, the rf magnetic field profiles are very similar to those of CP birdcages at similar frequencies. The coil is electrically balanced such that the electric potentials vanish on the central axial plane at both the LF and HF homogeneous modes.